Wanted: your ideas for a 2.0 v.d.c. 1-amp PSU

[Mark R]

I never got my multi one finished, but I like the look of that. Especially as I have a 20/30's cossor that needs one of these.

cheers Mark

[pwdrive]

Hello Al, if it does what you say on the can then spot on, I like the simple stuff, have you tried it on a receiver to see if ripple has any effect?

Lawrence

[Skywave]

No Lawrence, not yet; good thought though - must do that soon - a.k.a. the 'acid test' (pun not intended!)

Al.

[Mark Hennessy]

Looks good - nice and simple

Does that 470 ohm resistor provide enough current? Worst-case, a 2n3055 might have an HFE of only 20, implying that 50mA of base current would be needed at 1 amp. The 470 ohm resistor might be running at around 14mA, depending on the unregulated input. Something like 120 or 150 ohms might be better, but of course would warm up a bit. Although you're probably not suffering any problems with the circuit you've built, it's probably worth mentioning this as a potential issue for anyone that builds their own version in the future...

The 2n2219 is one of my favourite TO5 transistors. No idea how widely available they are these days, but it's probably worth saying (again, for the benefit of potential future constructors) that almost any NPN transistor will work there. I bet the humble BC108 would work well.

In a similar vein (for other constructors), it's probably worth pointing out that the diodes used in the regulator and crowbar circuit aren't critical, and any silicon diode will probably work. I like the 1n4148 as they are smaller and easier to handle - the 1n540x range are rather "chunky" and their leads fit tagstrip but not 0.1" Veroboard...

Should anyone want to improve the ripple performance of this then the 470 ohm resistor can be split in half, with a capacitor decoupling the mid-point. Even better; add another transistor to make the 2n3055 a Darlington pair, then larger values of resistance can be used - probably 4K7 or more - which will reduce the size of capacitor necessary. Also, a capacitor between the base of the 2n2219 and the output rail will reduce the ripple by increasing the AC feedback.

I've attached the schematic of the Hacker VP408 external power supply - you'll note a lot of commonality

Again, good work

Mark

[Skywave]

Thank you for your comments Mark. By and large, the only reason I chose many of those components that I did was because either they were 'conveniently to hand' on the work-bench or were devices that I've used before - and therefore familiar with their characteristics. As stated on the drawing, at present this is in the provisional / prototype stage, so input from yourself and any others is welcome.

Al.

[Skywave]

Amendment to drawing notice.

There was a drawing error in the circuit diagram as issued: the circuit (as shown) has now been edited. The 180-ohm resistor in series with the Vout pot was connected between the pot. track and Vout. This was incorrect. That resistor should be shown as between the pot. track on the 0v line. The amended drawing now shows this.

Apologies for any inconvenience thus caused.

Al / April 10th. //

[ed731pdh]

Just come off the phone from Dennis and (Dennis being Dennis) he chucked a nice sized elelctronic grenade in my direction. The early radios with 2 volt heaters were battery powered and the internal resistance of the batteries would limit the amount of cold amps being drawn by the valves. As he quite rightly pointed out, a modern equivalent would provide full amps from turning on and it is possible that this could limit the life of the 2 volt heaters, quite a good point and a question to which I'm stumped.

[Joe]

A thermistor would seem the obvious, though not necessarily the correct, solution.

- Joe

[Mark Hennessy]

Pretty easy to add a current limit - just one extra transistor - and perhaps some series resistance after the feedback loop, assuming we're able to find out what the internal resistance of a fresh battery would have been. It would be very easy to measure it if they still made them

[Skywave]

I would have thought that the internal resistance of a freshly-charged accumulator would have been in the range of 0.1 to 0.2 ohm.

I consulted a few of my vintage wireless text-books on this matter and discovered the following. A good, freshly-charged lead-acid accumulator had an open-circuit voltage of 2.1 v. At a heavy discharge current of a few amps., that voltage would be expected to fall to 2.0v after about half-an-hour. At the more usual discharge of about 1 amp., (which is the design rating for this eliminator), the 2.0v figure would have been reached after about 3 hours.

Hence, in this design, setting the Vout pot. for 2.1v. off-load, falling instantaneously to 2.0v. on-load, would appear to be not harmful to the valves.

Al.